The present invention relates to a communication system and, more particularly, to a communication system which frequently detects the state of each communication terminal and notifies the user of a given communication terminal upon detecting that a predetermined other communication terminal is set in a designated state.
This application is based on Japanese Patent Application No. 9-139772, filed May 29, 1997, the content of which is incorporated herein by reference.
In recent years, a known mobile communication system is constituted by portable information terminals (e.g., a cellular telephone, a PHS terminal: Personal Handy Phone System terminal, a pager, and a PDA: Personal Digital Assistants) the users carry to transmit/receive speech data and the like to/from another terminal such as a general home telephone, and by a base station connected to a communication line such as an ISDN line to communicate with the portable information terminals and connect them to the communication line.
In this mobile communication system, a public base station connected to a public switched telephone network and set indoors or outdoors at an unspecified location, a private base station connected to a switching unit (to be referred to as a PBX hereinafter) placed in a public facility (e.g., an office), or a private master unit set in a house and functioning as the master unit of a portable information terminal is used as a base station. The public base station, private base station and private master unit are referred to a master unit hereinafter. The portable information terminal (to be also referred to as a subsidiary unit hereinafter) is connected to the communication line through the public base station, the private base station, or the private master unit to communicate with the other party.
The portable information terminal is driven by a secondary battery or the like and used as a portable unit. In the service area of the master unit, the portable information terminal is connected to the communication line through the master unit connected via a wireless channel so that the portable information terminal can communicate with another terminal. Outside the service area of the master unit, the portable information terminal is connected to the communication line through an outdoor base station connected via a wireless channel to communicate with another terminal. Information to be transmitted/received by the portable information terminal includes speech data, text data, image data, and the like.
In the conventional mobile communication system, when a user wants contact from the other party when he/she has arrived at or left a certain place or he/she is set in a predetermined state, the user depends on the memory of the other party. If the other party forgets, no contact can be made. In this case, it is troublesome that the user must request the other party to make a call. The above problem, moreover, is not limited to a mobile communication system, but may also occur in a general communication system.